Metronidazole (metronidazole 250 mg) Dailymed

To reduce the development of drug-resistant bacteria and maintain the effectiveness of metronidazole tablets, USP and other antibacterial drugs, metronidazole tablets, USP should be used only to treat or prevent infections that are proven or strongly suspected to be caused by bacteria.

Warning

Metronidazole has been shown to be carcinogenic in mice and rats (see PRECAUTIONS ). Unnecessary use of the drug should be avoided. Its use should be reserved for the conditions described in the  INDICATIONS AND USAGE  section below.

Description

Metronidazole tablets, USP, 250 mg or 500 mg is an oral formulation of the synthetic nitroimidazole antimicrobial, 2-methyl-5-nitro-1H-imidazole-1-ethanol, which has the following structural formula:

   

Metronidazole 250 mg and 500 mg tablets, USP, for oral administration, contain the inactive ingredients: colloidal silicon dioxide, hydroxypropyl cellulose, lactose (anhydrous), microcrystalline cellulose, sodium starch glycolate, and stearic acid.

Clinical Pharmacology

Absorption

Disposition of metronidazole in the body is similar for both oral and intravenous dosage forms. Following oral administration, metronidazole is well absorbed, with peak plasma concentrations occurring between one and two hours after administration.

Plasma concentrations of metronidazole are proportional to the administered dose. Oral administration of 250 mg, 500 mg, or 2,000 mg produced peak plasma concentrations of 6 mcg/mL, 12 mcg/mL, and 40 mcg/mL, respectively. Studies reveal no significant bioavailability differences between males and females; however, because of weight differences, the resulting plasma levels in males are generally lower.

Distribution

Metronidazole is the major component appearing in the plasma, with lesser quantities of metabolites also being present. Less than 20% of the circulating metronidazole is bound to plasma proteins. Metronidazole appears in cerebrospinal fluid, saliva, and breast milk in concentrations similar to those found in plasma. Bactericidal concentrations of metronidazole have also been detected in pus from hepatic abscesses.

Metabolism/Excretion

The major route of elimination of metronidazole and its metabolites is via the urine (60% to 80% of the dose), with fecal excretion accounting for 6% to 15% of the dose. The metabolites that appear in the urine result primarily from side-chain oxidation [1-(ß‑ hydroxyethyl)-2-hydroxymethyl-5-nitroimidazole and 2-methyl-5-nitroimidazole-1-yl‑ acetic acid] and glucuronide conjugation, with unchanged metronidazole accounting for approximately 20% of the total. Both the parent compound and the hydroxyl metabolite possess in vitro antimicrobial activity.

Renal clearance of metronidazole is approximately 10 mL/min/1.73 m2. The average elimination half-life of metronidazole in healthy subjects is eight hours.

Renal Impairment

Decreased renal function does not alter the single-dose pharmacokinetics of metronidazole. Subjects with end-stage renal disease (ESRD; CLCR=8.1±9.1 mL/min) and who received a single intravenous infusion of metronidazole 500 mg had no significant change in metronidazole pharmacokinetics but had 2-fold higher C _max of hydroxy-metronidazole and 5-fold higher C _max of metronidazole acetate,compared to healthy subjects with normal renal function (CL _CR=126±16 mL/min).Thus, on account of the potential accumulation of metronidazole metabolites in ESRD patients, monitoring for metronidazole associated adverse events is recommended (see P R E CAU T ION S ).

Effect of Dialysis

Following a single intravenous infusion or oral dose of metronidazole 500 mg, the clearance of metronidazole was investigated in ESRD subjects undergoing hemodialysis or continuous ambulatory peritoneal dialysis (CAPD). A hemodialysis session lasting for 4 to 8 hours removed 40% to 65% of the administered metronidazole dose, depending on the type of dialyzer membrane used and the duration of the dialysis session. If the administration of metronidazole cannot be separated from the dialysis session, supplementation of metronidazole dose following hemodialysis should be considered (see DO S A G E AND A D M I NI ST RA T ION ). A peritoneal dialysis session lasting for 7.5 hours removed approximately 10% of the administered metronidazole dose. No adjustment in metronidazole dose is needed in ESRD patients undergoing CAPD.

Hepatic Impairment

Following a single intravenous infusion of 500 mg metronidazole, the mean AUC ₂₄ of metronidazole was higher by 114% in patients with severe (Child-Pugh C) hepatic impairment, and by 54% and 53% in patients with mild (Child-Pugh A) and moderate (Child-Pugh B) hepatic impairment, respectively, compared to healthy control subjects. There were no significant changes in the AUC ₂₄ of hydroxyl-metronidazole in these hepatically impaired patients. A reduction in metronidazole dosage by 50% is recommended in patients with severe (Child-Pugh C)hepatic impairment (see DO S A G E AND A D M INI ST RA T ION ). No dosage adjustment is needed for patients with mild to moderate hepatic impairment. Patients with mild to moderate hepatic impairment should be monitored for metronidazole associated adverse events (see  P R E C A U T IONS  and DO S A G E AND A D M I NI ST RA T ION ).

Geriatric Patients

Following a single 500 mg oral or IV dose of metronidazole, subjects greater than 70 years old with no apparent renal or hepatic dysfunction had a 40% to 80% higher mean AUC of hydroxy-metronidazole (active metabolite), with no apparent increase in the mean AUC of metronidazole (parent compound), compared to young healthy controls less than 40 years old. In geriatric patients, monitoring for metronidazole associated adverse events is recommended (see PRECAUTIONS ).

Pediatric Patients

 In one study, newborn infants appeared to demonstrate diminished capacity to eliminate metronidazole. The elimination half-life, measured during the first 3 days of life, was inversely related to gestational age. In infants whose gestational ages were between 28 and 40 weeks, the corresponding elimination half-lives ranged from 109 to 22.5 hours.

Microbiology

Metronidazole, a nitroimidazole, exerts antibacterial effects in an anaerobic environment against most obligate anaerobes. Once metronidazole enters the organism by passive diffusion and activated in the cytoplasm of susceptible anaerobic bacteria, it is reduced; this process includes intracellular electron transport proteins such as ferredoxin, transfer of an electron to the nitro group of the metronidazole, and formation of a short-lived nitroso free radical. Because of this alteration of the metronidazole molecule, a concentration gradient is created and maintained which promotes the drug’s intracellular transport. The reduced form of metronidazole and free radicals can interact with DNA leading to inhibition of DNA synthesis and DNA degradation leading to death of the bacteria. The precise mechanism of action of metronidazole is unclear. 

Drug Resistance A potential for development of resistance exists against metronidazole. 

Resistance may be due to multiple mechanisms that include decreased uptake of the drug, altered reduction efficiency, overexpression of the efflux pumps, inactivation of the drug, and/or increased DNA damage repair. 

Metronidazole does not possess any clinically relevant activity against facultative anaerobes or obligate aerobes.

Activity In Vitro and in Clinical Infections

Metronidazole has been shown to be active against most isolates of the following bacteria both in vitro and in clinical infections as described in the  INDICATIONS AND USAGE  section.

Gram-positive anaerobes Clostridium species Eubacterium species e Peptococcus species Peptostreptococcus species

Gram-negative anaerobes Bacteroides fragilis group ( B. fragilis, B. distasonis, B. ovatus, B. thetaiotaomicron,   B.vulgatus)   Fusobacterium species

Protozoal parasites Entamoeba histolytica Trichomonas vaginalis

The following in vitro data are available, but their clinical significance is unknown:

Metronidazole exhibits in vitro minimal inhibitory concentrations (MIC’s) of 8 mcg/mL or less against most (greater than or equal to 90%) isolates of the following bacteria; however, the safety and effectiveness of metronidazole in treating clinical infections due to these bacteria have not been established in adequate and well-controlled clinical trials.

Gram-negative anaerobes Bacteroides fragilis group ( B. caccae, B. uniformis)   Prevotella species ( P. bivia, P. buccae, P. disiens)  

Susceptibility Tests: When available, the clinical microbiology laboratory should provide results of in vitro susceptibility test results for antimicrobial drug products used in resident hospitals to the physician as periodic reports that describe the susceptibility profile of nosocomial or community-acquired pathogens. These reports should aid the physician in selecting an antibacterial drug product for treatment.

Quantitative methods are used to determine antimicrobial inhibitory concentrations  (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. For anaerobic bacteria, the susceptibility to metronidazole can be determined by the reference broth and/or agar method ^1,2.  

The MIC values should be interpreted according to the criteria provided in the following Table

Susceptibility Test Interpretive Criteria for Metronidazole against Anaerobes*†

MIC (mcg/mL)      Interpretation
MIC (mcg/mL)	Interpretation
≤ 8	sceptible (S)
16	Intermediate (I)
≥ 32	Resistant (R)
*Agar dilution method is recommended for all anaerobes †Broth dilution method is recommended for testing of Bacteroides fragilis group only; for this group, MIC values by agar and broth dilution methods are considered equivalent

A report of “Susceptible” (S) indicates that the antimicrobial is likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentrations at the infection site necessary to inhibit growth of the pathogen. A report of “Intermediate” (I) implies that an infection due to the isolate may be appropriately treated in the body sites where the drugs are physiologically concentrated or when a high dosage of drug is used. A report of “Resistant” (R)  indicates that the antimicrobial is not likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentration usually achievable at the infection site; other therapy should be selected.

Quality Control

Standardized susceptibility test procedures require the use of laboratory controls to monitor and ensure the accuracy and precision of supplies and reagents used in the assay, and the techniques of the individuals performing the test. ^1,2 Standard metronidazole powder should provide a value within the MIC ranges noted in the following table:

Acceptable Quality Control Ranges for Metronidazole against Anaerobes

Quality strain	Minimum Inhibitory Concentration (mcg/mL)
	Agar	Broth
Bacteroides fragilis ATCC 25285	0.25–1.0	0.25-2.0
Bacteroides thetaiotaomicron ATCC 29741	0.5–2.0	0.5-4.0
Clostridium difficile ATCC 700057	0.125-0.5	-
Eggerthella lenta ATCC 43055	-	0.125-0.5

For protozoal parasites: Standardized tests do not exist for use in clinical microbiology laboratories.